For perishable packaged products, exposure to the air outside the package can adversely effect the quality or shelf life of the product. For example, cigarettes exposed to the air lose moisture and can taste stale or harsh within a short time.
For this reason, the packages of perishable goods like cigarettes are typically wrapped in an air-tight overwrap. The overwrap prevents moisture loss and maintains product freshness. A material commonly used for this purpose is polyethylene film, which is wrapped around the package and sealed by heating and pressing together overlapping portions of the wrap.
Because seal quality is important to product quality, its measurement is an important part of quality control in the food and tobacco industries. The quality of an overwrap seal is a function of its resistance to air leakage. Therefore, means and methods for easily and accurately testing air leakage from overwraps are important to these industries.
The most popular means of testing air leakage involves introducing pressurized air into the package and measuring in some way the pressure that builds up inside. In one method, the operator punctures the overwrap and package with a needle through which air flows into the inside of the package. This procedure has several problems. The most important problem is that the needle breaches the integrity of the package. As a result, the package now has a visible hole. This causes the contents inside the tested package to be exposed directly to the air outside, precisely the situation packages are designed to avoid.
A more troubling problem with this method has to do with consumer confidence: a punctured package appears to have been tampered with. Due to recent and well-publicized incidents of product tampering in various industries, consumers have become particularly sensitive to the integrity of packaged goods. Manufacturers who test package seals by the puncture method risk causing consumer alarm if such a tested package was ever accidentally shipped to a retailer and examined by a consumer.
Devices which introduce pressure through a needle also have problems accurately introducing or measuring pressure. The air pressure introduced can vary depending upon whether the needle enters an air space in the package or the packaged material. In the case of a cigarette pack, for example, this method would give different results if the needle entered either an air space or a cigarette rod. Furthermore, pressure could leak out from around the punctured hole.
These devices suffer from other problems as well. Some are too slow or too complex to be useful on the manufacturing floor for providing real time information on seal quality of randomly selected packages. Yet other methods require sources of special gases, fluids, or highly pressurized air.
A more sophisticated device is the package leak tester of Hester et al., U.S. Pat. No. 4,539,836 ("Hester"). Hester's device requires the operator to puncture a hole through the overwrap into the package, thus breaching package integrity. However, the Hester device introduces air pressure to the package by means of a test head. The test head contains a concentric vacuum area which surrounds a central area through which pressure is delivered. The vacuum area holds the package in place and the central area delivers pressure to the hole in package. If the overwrap seal is tight, the pressure to the package will be greater than the vacuum holding the package to the test head, and the package will be ejected.
This device also has several problems in addition to some of the ones associated with the puncture method. First, there are problems of accuracy. The vacuum between the overwrap and test head is maintained by rubber O-rings. O-rings have a danger of leaking. One result of this is decreased vacuum to the package and a consequent indication that the overwrap is tighter than it actually is. Another problem is that air pressure meant to be delivered into the overwrap might instead get sucked into the vacuum area. This would indicate a weak seal since there would not be enough pressure to eject the package. Yet another problem is that Hester's device introduces air to the inside of the package through the hole created by puncture, not just under the overwrap. This may affect the accuracy of measurement. It will surely extend the time necessary for measurement since the pressure will have to equilibrate inside the package as well as between the package and overwrap. This could make the measurement of large packages troublesome since a lot of air would be needed to completely pressurize them.
Second, Hester's device does not quantitatively measure the seal quality: either the test head holds the package, or the package is ejected.
Third, the device is practical only for small packages. It relies on suction to hold the weight of the package and pressure to eject it from the test head. It would be difficult to generate the enormous vacuums and pressures necessary to hold and eject heavy packages, and such pressures could rip the overwrap.
Finally, the device is awkward. It requires an outside source of air pressure and so is not self-contained.
Therefore, there is a need in the art to provide for an instrument to measure package seals which does not breach the integrity of the package, which can accurately and quantitatively measure pressure, which avoids air leaks around the measuring head and into the package, which can accommodate packages of many sizes, and which is portable and which can be used quickly and easily at various locations on the manufacturing floor (e.g., at an inspection station in the production area).